So, I have some customer highlights that I created long ago... They work fine in the Desktop and iOS apps. But, in the Web App, they do not show up. I can't select the custom highlights from the web app. I have to from the desktop or iOS app.
However, by some whim, I selected another highlight in the Web App for another word in the same area within several paragraphs of the ones I had highlighted with custom highlights, and voila, the other, custom, highlights suddenly appeared.